1. Field of the Invention
The present invention relates to a recycling system for a semiconductor factory, and in particular to a recycling system and method for recycling a photoresist developer solution containing tetra-methyl-ammonia hydroxide (TMAH).
2. Description of the Related Art
In semiconductor fabrication, photolithography comprises the exposure and development of a photoresist layer overlying a semiconductor substrate to define a pattern thereon. Conventionally, the exposed acidic organic photoresist is neutralized and dissolved with basic developer and the unexposed photoresist pattern remains as a mask. Tetra-methyl-ammonia hydroxide (TMAH) is a widely used basic photoresist developer.
Conventionally, semiconductor factories recycle and reuse the basic TMAH developer by way of a recycling system. A conventional system utilizes a conductivity meter or a UV spectrometer to measure the concentration of TMAH in the recycled solution. The recycled solution is then adjusted to a predetermined alkaline level according to the measured concentration by adding additional TMAH thereto. The adjusted TMAH developer is then used as the photoresist developer of subsequent photolithography. After several recycles, the recycled TMAH developer is discharged to a waste system.
The quality of developer solution depends on the concentration of TMAH. However, the concentration of TMAH is influenced by ions in the recycled solution. For example, the neutralized compounds of photoresist and developer are ionized to generate OR1xe2x88x92, OR2xe2x88x92, or (CH3)4NOR+ and the remaining metal ions Al3+, Mo2+, K+, or Na+ on the semiconductor substrates are rinsed in the developer. When a conductivity meter is utilized, the measured conductivity b % of a recycled developer solution is higher than the real conductivity a % due to the recycled solution containing ions other than basic TMAH. Since the TMAH concentration in the recycled solution exceeds estimated, the alkalinity, i.e. TMAH concentration, of the adjusted recycled solution is inaccurate.
When a UV spectrometer is utilized for TMAH adjustment, the absorption value of the recycled TMAH is influenced by the photoresist in the recycled solution. The measured absorption value is higher than the real absorption value of the TMAH therein. Similarly, the alkalinity of adjusted recycled solution is inaccurate.
It is difficult to eliminate the influence of other ions from the recycled solution using conductivity meter or UV spectrometer. Thus the accuracy of the adjusted TMAH concentration and the quality of subsequent photolithography are reduced.
A solution is to utilize high accuracy ion chromatography (IC) to isolate TMAH from the recycled solution for concentration determination, eliminating the influence of other ions and thus the exact concentration of TMAH is represented. The drawback of IC is its laboratory scale and batch-process concentration determination, making it unsuitable for continuous and long-term in-line analysis of TMAH recycling systems. In addition, the cost of materials used in ion chromatography and maintenance thereof are too high to be utilized in TMAH recycling systems.
The object of the present invention is to provide a rapid, economic, and accurate TMAH recycling system and method thereof able to measure TMAH concentration in the recycling system accurately such that alkalinity of TMAH in the recycled developer solution can be controlled.
The present invention provides a method for adjusting tetra-methyl-ammonia hydroxide (TMAH) concentration for recycling. M wavelengths are pre-selected between 220 nm and 250 nm, wherein m is equal to or greater than 2. Absorption values A1, Y1 to Ym of a recycled developer solution at wavelength 210 nm and m wavelengths between 220 nm and 250 nm are measured respectively. Y1 to Ym are input to an nth-degree polynomial to generate a wavelength-absorption relationship Y=C1Xn+ . . . +Cnxe2x88x921X+Cn, wherein X is wavelength, n is a positive integer and C1 to Cn are coefficients of the relationship. Wavelength 210 nm is input into the wavelength-absorption relationship to generate an absorption value Y210. A difference A3 between A1 and Y210 is calculated as the absorption value of TMAH in the developer solution and A3 is then input to an absorption calibration curve of TMAH at 210 nm to generate a corresponding TMAH concentration. TMAH is then added into the recycled developer solution according to the corresponding TMAH concentration.
A recycling system of a developer solution containing TMAH is further provided according to the present invention. The recycling system comprises a recycle tank to collect the recycled developer solution from a photoresist development system via a recycle pipeline. An adjustment tank is coupled to the recycle tank for loading with highly concentrated TMAH and connected to the recycle tank with an adjustment pipeline. A spectrometer is coupled to the recycle tank for measuring absorption values of the developer solution in the recycle tank. A processor connects to the spectrometer and the adjustment pipeline, programmed to calculate a TMAH concentration in the recycle tank according to the measured absorption values from the spectrometer and delivering an amount of highly concentrated TMAH from the adjustment pipeline to the recycle tank to achieve a desired TMAH concentration according to the calculated TMAH concentration. The processor is programmed to calculate the TMAH concentration in the recycle tank by the following steps. Absorption values Y1 to Ym on m wavelengths between 220 nm and 250 nm of the recycled developer solution are read respectively, wherein m is equal to or larger than 2, and an absorption value A1 on 210 nm is also read. Y1 to Ym are then input to an nth-degree polynomial to generate a wavelength-absorption relationship Y=C1Xn+ . . . +Cnxe2x88x921X+Cn, wherein X is wavelength, n is a positive integer and C1 to Cn are coefficients of the relationship. Wavelength 210 nm is input into the wavelength-absorption relationship to generate an absorption value Y210. A difference A3 between the A1 and Y210 is calculated as the absorption value of TMAH in the developer solution. A3 is input to an absorption calibration curve of TMAH at 210 nm to generate a corresponding TMAH concentration in the recycle tank.
When a recycling system achieves stability, the present invention further provides a simplified method for recycling a developer solution containing tetramethyl-ammonia hydroxide (TMAH). Absorption values A1 and A2 of the recycled developer solution are measured at wavelengths 210 nm and 220 nm. Absorption value A3 of TMAH in the developer solution is calculated as A3=A1xe2x88x92A2xc3x97Co, wherein Co=(A1xe2x80x2xe2x88x92A3xe2x80x2)/A2xe2x80x2, A1xe2x80x2 and A2xe2x80x2 are absorption values of a recycled developer solution with known TMAH concentration at wavelengths 210 nm and 220 nm respectively, and A3xe2x80x2 is the standard absorption value of the known TMAH concentration at 210 nm. A3 is input to an absorption calibration curve of TMAH at 210 nm to generate a corresponding TMAH concentration. TMAH is added into the recycled developer solution accordingly, thereby achieving a desired TMAH concentration.
The present invention also provides a processor utilized in the above recycling system, programmed to calculate the TMAH concentration in the recycle tank by the following steps. Absorption values A1 and A2 of the recycled developer solution at wavelengths 210 nm and 220 nm are read. An absorption value A3 of TMAH in the developer solution is calculated by A3=A1xe2x88x92A2xc3x97Co, wherein Co=(A1xe2x80x2xe2x88x92A3xe2x80x2)/A2xe2x80x2, A1xe2x80x2 and A2xe2x80x2 are absorption values of a recycled developer solution with known TMAH concentration at wavelengths 210 nm and 220 nm respectively, and A3xe2x80x2 is the standard absorption value of the known TMAH concentration at 210 nm. A3 is input to an absorption calibration curve of TMAH at 210 nm to generate a corresponding TMAH concentration in the recycle tank.
According to the present invention, a low cost and highly stable spectrometer is utilized by way of a processor, such as a computer, with which TMAH concentration in the recycle tank is computed and adjusted accurately.
A detailed description is given in the following embodiments with reference to the accompanying drawings.